Antenna towers have long been used for supporting antennas used in wireless communication networks, such as cellular communications systems. One common type of antenna tower is constructed of an interconnected lattice framework of steel beams. Another common type of antenna tower is a monopole tower consisting of a single tubular mast or pole extending upwardly from ground level. Monopole towers have grown in popularity because the visual impact of monopole towers is less than that of lattice-type towers and because of the relatively low cost as compared with lattice-type towers.
Wireless communication networks are divided into cells each arranged to communicate with mobile stations with minimal interference and that, in the aggregate, define a coverage area. A mobile station traversing the coverage area has its communications handed-off between adjacent cells. Each cell includes one or more individual antennas arranged and combined in a manner to communicate with a mobile station. Each antenna consists of multiple radiating elements that are housed within an outer housing, which may have a rectangular, box-like shape, that is affixed to a triangular support platform mounted to the monopole tower.
Changes in wireless coverage are accomplished by changing a main beam direction of the antenna. In most wireless communication networks, the main beam direction may be changed by an elevational or azimuthal adjustment after the antennas have been installed on the antenna tower. The main beam direction may be adjusted for varying the coverage area of each cell as the number of customers increases and additional cells are added to accommodate increasing numbers of mobile stations. The main beam direction may also be adjusted to compensate for new adjacent construction, vegetation growth, or other changes in the surrounding environment of the monopole tower.
One method for altering the main beam direction of the radiation pattern is to physically relocate the antennas and/or direction or to replace the antennas with certain fixed radiation characteristics with antennas having different fixed radiation characteristics. However, such physical relocation or replacement is difficult. Another method for altering the coverage is to mount the antennas to the antenna tower with brackets that allow mechanical adjustment of the downtilt of the individual antennas. However, service personnel must adjust the main beam direction of the antennas by climbing the tower to a service platform near the antennas or by being supported from an elevated lifting device such as a cherry picker. Not only is this costly, but wireless communications service is interrupted while the manual adjustment of the downtilt is being performed.
Operators of wireless communication networks typically need to obtain permission from residential and zoning boards to erect antenna towers. Antenna towers are by their very nature prominent structures. The preferred locations for antenna towers are the most visible locations relative to the surrounding landscape within the intended coverage area. Conventional monopole towers with triangular support platforms have an appearance that, while less objectionable than lattice-type towers, is not aesthetically pleasing. As a result, permission to erect an unsightly monopole tower may be difficult to obtain in urban and suburban venues. One approach for overcoming zoning opposition is to disguise or otherwise conceal the antennas and supporting platforms of the monopole tower to lessen the visual impact. For example, the monopole tower may be adorned with structures emulating foliage such that, to a casual observer, the tower resembles a tree or other vegetation. However, such camouflaging structures are impractical, difficult and expensive to construct, and costly to maintain.
Each wireless telephony provider in a geographical area requires their own dedicated cells to provide coverage. As a result, each provider will position their own set of towers in suitable sites within the geographic area. Because suitable sites are increasingly difficult to secure, more complex and visually objectionable antenna arrangements are being deployed to maximize coverage in the geographic area. In particular, the usage of the monopole tower may be increased by permitting multiple operators to share a single monopole tower. To that end, multiple operators may be accommodated by attaching additional triangular support platforms to the monopole tower and providing each platform with an additional set of antennas.
The number of antennas required to service multiple providers may be further reduced by diplexing individual providers on the same antennas. However, combining providers on a single antenna increases the likelihood of intermodulation distortion. In addition, the installation process for diplexed systems becomes more critical as, for example, a poorly-made jumper, a dirty connector or an improperly torqued connector may degrade performance. As the number of antennas servicing each antenna is limited, the ability to correct an antenna failure by simply changing to a spare antenna is limited. Furthermore, the duplexer adds losses that reduce coverage. Moreover, the coverage area for diplexed providers is identical and, as a result, variations in the main beam direction must be mutually agreed upon. Specifically, the main beam directions for two providers sharing antennas are not independently adjustable. Finally, the diplexing equipment is expensive and adds significantly to the system cost.
Therefore, it would be desirable to construct a monopole tower having antennas arranged to accommodate multiple carriers or providers, and yet which presents a reduced visual impact and affords independent control of the respective coverage areas.